Planet

Planets are objects of a certain type that orbit stars. Objects achieve this rank if they conform to a specific set of parameters including roundness and controlling their orbit gravitationally, but these parameters are debatable and vary much throughout cultures. Planets could be small and icy, far from their star, they could be composed of gas and have massive atmospheres, they could be fireballs orbiting close to their home star, they could be small, rocky and moderate in temperature, and everything in between. There are millions of planets in most galaxies, with many of them being able to support life in some form.

Humans have used a classification system for planetary bodies since the early 21st century, with Class M (or mesplanet) being a planet capable of sustaining and/or evolving life similar to themselves.

Classification system
Humanity had constructed a basic classifacation system for planets by the time they developed warp travel in the late 21st century. This system continued to be modified over the years, but stayed fundamentaly the same for a very long time.

Class A- Geothermal
Class A planets are small, geologically active worlds usually just formed from the protoplanetary disk. Once it cools down sufficiently, it usually becomes a class C or B.

Class B- Geomorteus
Class B planets are hot, rocky worlds that form very close to their star.

Class C- Geoinactive
Class C planets are cooled Class A worlds. They can be found in any place in the star system, and are very much dead, with no geological activity.

Class D- Dwarf
Dwarf planets are not technically planets; they are small and icy and usually have erratic orbits. They almost always form in the far reaches of the star system, but they occasionly are created in asteroid belts.

Class E- Geoplastic
Class E planets are cooled Class A planets where the world has started to evolve into a Class M, H, O, or K. Its surface is still mostly molten though, and it has to cool more before it can evolve further.

Class F- Geometallic
After a Class E planet cools enought, it becomes a Class F. Class F planets are still volcanically active, with volcanoes releasing gases that may form oceans and proto-life chemicals. Once most of the volcanic activity ceases, the planet has become class G.

Class G- Geocrystalline
A Class G planet is a stabilized Class F where a basic atmosphere has formed and basic life forms may have begun to evolve. These planets have the potential to become a Class M, H, K,O, P, N, R or Q planet.

Class H- Desert
Class H planets are worlds where their is very little liquid on the surface. They are often, but do not have to be, hot, and also often have a sandy or rocky surface. Life does evolve on this class sometimes, but it is usually not humanoid.

Class I- Uranian
Class I planets, commomly called "ice giants", are medium sized gas based planets that form on or beyond the outer edge of the stars habitable zone. Their atmospheres usually consist of methane, helium, water vapor and ammonia. They sometimes have small rocky cores that could become a Class X.

Class J- Jovian
Jovian planets are large, gas based worlds consisting of hydrogen, helium and some other trace gasses. If they have a core, it is usually liquid based rather than solid.

Class K- Martian
Class K planets have a thin, usually carbon dioxide atmosphere and no liquid water, and are sometimes formed from a Class M or O planet that has been stripped of its atmosphere. The only life that could evolve on these planets are simple single celled organisms.

Class M- Mesoplanet
Mesoplanets are "Earth-like" worlds where less than 75% of the surface is covered by water and temperatures are mild and stable. They are usually found in the middle of a stars habitable zone. Class M planets are very commonly suitable for the evolution of life.

Class N- Venusian
Class N planets are worlds where the greenhouse effect has created a chain reaction, choking the atmosphere with poisonous gasses and causing temperatures to soar to 500 degrees Celsius and above. They are quite similar to Class Y planets, and are more like a specific type of Class Y than a class of their own.

Class O- Oceanic
Oceanic planets are worlds where over 75% of the surface is covered by water, and maybe water ice. These planets can be either warm or cold, but they usually form in the inner edge of their stars habitable zone.

Class P- Icy
Class P planets are planets where a large percentage of the surface is covered by ice and snow. They are usually found orbiting in the outer edge of their stars habitable zone.

Class Q- Erratic
Class Q planets have very eccentric orbits, or are near variable stars. The climate is changing constantly, so it is quite difficult for life to form on these worlds.

Class R- Rouge
Rouge planets are planets of any class that formed within a star system, but were expelled because of a massive collision or gravitational "slingshot". They are occasionaly geothemically active, and hence may have enough heat to capable of sustaining life.

Class S- Supergiant
Supergiants are similar to Class J planets, exept that their cores have sufficient pressure to make hyrdogen and other gasses become liquid or even metallic.

Class U- Ultragiant
Ultragiants are massive planets that form in many star systems, mostly absorbing all of the protoplanetary disk. They can also be referred to as "failed stars"; if a Class U planet was any larger, its gravity could ignite a nuclear fusion reaction in its core, turning it into a star. If an ultragiant is close enough to its star, it will become extremely hot and the atmosphere may start "blowing" off due to solar wind. These planets are called Hot Jupiters. In the early 21st century, the planets humans had discovered were mostly Hot Jupiters.

Class X- Chthonian
Chthonian planets are failed gas giants; their thick atmospheres have been stripped away revealing an icy, metallic or rocky inner core.

Class Y- Uninhabitable
Generally completely uninhabitalble and unfriendly to life in some way, whether it be from a poisonous atmosphere or bombardment from meteorites or a radioactive crust. They are uncommon in the galaxy; a planet, to achieve this class, must not fit within any other class. Hence, planets of this class are usually astronomical oddities worth study.

Thanks to Star Trek: The Final Frontier fan fiction website for the basics of this classification system: http://www.sttff.net/index.html